A communication system transfers information between a source and a destination. Generally, a communication system includes a transmitter and a receiver which transmit and receive information signals over some media, respectively. This media may be cable wiring or atmosphere. When communications occur over atmosphere, or air waves, they are commonly referred to as "wireless" communications. Examples of wireless communications systems include digital cellular, packet data paging and digital cordless telephones, wireless modems, wireless local and wide area networks, digital satellite communications and personal communications networks.
A new communication system, termed "Multicarrier QFSK" (for Quadrature Frequency Shift Keying), is being developed for use in pagers and other portable equipment. In one QFSK system, four separate bands are arranged about a center frequency f.sub.c. FIG. 1 illustrates an example receiving spectrum. Referring to FIG. 1, each of the four bands A-D carries separate signals in a bandwidth of about 8 KHz. The two bands adjacent to f.sub.c have a center frequency spaced 5 KHz from f.sub.c, while the two outside bands have a center frequency spaced about 15 KHz from f.sub.c. The frequency deviations for each channel are +/-800 Hz and +/-2400 Hz at a data rate of 3200 bps. One existing QFSK system that uses four separate bands is commonly referred to as REFLEX50.
To effectively use a QFSK system, the four subchannels must first be separated. One problem is how to separate the four subchannels with the low power consumption necessary for effective use in portable equipment such as pagers.
A conventional separation solution would be to use a I/O mixer that takes a signal and divides it into 20 KHz bands on either side of f.sub.c to provide two signals, the I and Q signals. A conventional system would apply digital signal processing (DSP) techniques to each of the I and Q signals to obtain the output signals. Particularly, because each signal (the I signal and the Q signal) contain two separate signals, these signals would require separating, and a DSP is a conventional way to separate such signals. This solution is acceptable for systems in which power consumption is relatively unimportant, such as desk top systems. However, for low power consumption in portable systems, DSP channel separation systems consume an unacceptable amount of power. Furthermore, dedicated hardware for such a DSP system can be prohibitively expensive. Another separation system that would provide low power consumption and low cost would be useful.
Moreover, many QFSK systems utilize a single carrier FSK modulation, such as the REFLEX25 system. It would be desirable to provide a solution capable of use in systems using a single channel or multiple sub-channels.